1. Field of the Invention
The present invention relates to a multiplexed digital communication system for transmitting channel identification information, and particularly to a multiplex communication system which enables a desired channel to be selected by users from frequency-multiplex or time-division-multiplex channels.
2. Description of the Prior Art
It is known to utilize frequency-division multiplex technology and time-division-multiplex technology for transmitting a multichannel video signal to a receiving side from a transmitting side.
FIG. 1 is a block diagram indicating the construction of a frequency division multiplex coherent optical communication system, disclosed in "Beat Count Tuning Method for Optical Frequency-Division-Multiplex Transmission" by Kitajima, et al., "Proceedings of the Spring National Convention of the Electronics, Information and Communication Society in Japan, 1990", Article No. B-997.
In this figure, the reference numeral 1 denotes a signal beam of a certain channel; 2 a local oscillation frequency beam; 3 a photo coupler; 4 a photo sensor; 5 an intermediate frequency amplifier; 6 a demodulator; 7 a variable wavelength LD (laser diode) for outputting local oscillation frequency beam 2; 8 a drive circuit for driving LD 7; 9 an AFC (Automatic Frequency Control) circuit; 10 a switch; 11 a channel number input terminal; 12 a bias circuit; 13 a digital comparator; 14 a selected channel counter; and 15 a beat detector. Digital comparator 13 is provided with a register at an input terminal.
An operation of the system will next be explained. When a desired channel has been selected, switch 10 is closed and AFC circuit 9 controls a current to be applied to variable wavelength LD 7 in order to stabilize the intermediate frequency. When a channel number to be selected is input to digital comparator 13 through input terminal 11, a difference between the currently selected channel number and the target channel number is output from digital comparator 13. Depending on this output, switch 10 is turned OFF and a bias signal applied to LD 7 is scanned toward the target channel by bias circuit 12. Beat detector 15 outputs a pulse signal corresponding to the number of channels to be scanned until the target channel is obtained. Selected channel counter 14 counts the pulse signals sent from beat detector 15. When the channel number counted by beat detector 15 becomes identical to the target channel number, switch 10 is turned ON by an output from digital comparator 13 and the scanning of the bias signal is stopped. Finally, the intermediate frequency of the target channel is locked by AFC circuit 9, thereby ensuring stable reception of the target channel.
Next, the channel identification in a TDM system will be explained. FIG. 2 indicates a frame format in the time division multiplex communication system disclosed in the Japanese Patent Public Disclosure No. 91534/1989. A frame is composed of eight subframes and the leading one word of each subframe is used as frame synchronization channel (H channel). Identification information (ID channel) of the frame synchronization channel is inserted in unused seven bits of each H channel. When the channel identification information inserted in the transmitting side is read in the receiving side, it can be detected which channel is currently being received.
Therefore, a desired channel can be finally received by sequentially reading the channels to control the read operation to reduce a difference between receiving and target channel numbers.
FIG. 3 is a block diagram of the construction of the time-division-multiplex video signal transmission system disclosed on pages 9 to 14 of the 3rd Optical Communication System Symposium Material (edited by Authorized Committee for Optical Communication System Research of the Electron, Information and Communication Society in Japan, December 1989).
In FIG. 3, the reference numerals 20a-20x denote a plurality number N of A/D converters for converting multi-channel analog video signals (#1, #2, . . . , #N) sent from the head end to digital video signals. These converters comprise low-pass filters for removing a high frequency spectrum which is not related to the video signal. The reference numeral 21 denotes a TDM multiplexing apparatus which time-division-multiplexes the outputs of A/D converters 20a-20x; 22 a TDM signal demultiplexing apparatus which executes functions reverse to those of apparatus 21; 23a-23x a plurality number N of D/A converters for converting the demultiplexed digital video signals of the respective channels to analog video signals; 24a-24x N modulators for frequency-modulating the analog signals; 25 an FDM multiplexing apparatus for generating a frequency-division-multiplex signal from the outputs of N modulators 24a-24x; 26 a video signal receiver; 27 a channel decoder for generating a channel selection signal; and 28 an input unit for channel selection.
FIG. 4 is a diagram showing in detail constructions of TDM multiplexing apparatus 21 and TDM signal demultiplexing apparatus 22. In FIG. 4, 30a-30x are N input buffers provided for the respective channels; 31 a shift register for converting a parallel signal input from the buffers to a serial signal; 32 a clock generator; 33 a frame synchronization pattern inserter; 34 a frame synchronization pattern generator; 40 a shift register for converting a serial input signal to a parallel signal; 41 a clock generator for extracting a clock element from an input signal; 42 a frame counter; 43a-43x AND circuits for controlling the timing to output the demultiplexed signals; 44a-44x output buffers;.45 a frame synchronization pattern detector; and 46 a frame synchronization circuit.
An operation of the system will next be explained.
As shown in FIG. 3, in the case of multiplexing, for example, N-channel video signals, parallel N-channel input video signals are respectively converted to digital signals by M-bit A/D converters 20a-20x. These digital signals are then converted to a time-division-multiplex signal by TDM apparatus 21 and are transmitted to the transmission line as a serial video signal. On the other hand, the received video signal is separated into each channel by TDM signal demultiplexing apparatus 22. The demultiplexed video signals of the respective channels are input to M-bit D/A converters 23a-23x and then converted to analog signals. These analog signals are then frequency-modulated by modulators 24a-24x, frequency-division multiplexed by FDM apparatus 25, and then sent to video signal receiver 26 as RF signals. A desired video signal can be obtained in video signal receiver 26 as explained hereunder. An input signal from input unit 28 is decoded to a channel selection signal by channel decoder 27. An oscillation frequency of a local oscillation circuit within video signal receiver 26 is changed depending on the channel selection signal to tune to one channel of the received RF signals. In this manner, the baseband signal of the selected channel is regenerated and a video signal is displayed on receiver 26.
In TDM multiplexing apparatus 21 and TDM signal demultiplexing apparatus 22 shown in FIG. 4, pieces of data of the respective channels are input in parallel to shift register 31 through input buffers 30a-30x in the transmitting side. The contents of shift register 31 are read sequentially from channel #1 to channel #N under the control of clocks output from clock generator 32. A frame synchronization pattern generated in frame synchronization pattern generator 34 is inserted in the output of shift register 31 in frame synchronization pattern inserter 33 and a TDM signal thus prepared is then transmitted to the transmission line as a serial signal.
In the receiving side, a received serial signal is input to shift register 40. The contents of the shift register 40 are sequentially shifted under the control of clocks output from clock generator 41. When the data of the respective channels have been shifted to the normal position, an output-enable signal is output from frame counter 42 and AND circuits 43a-43x open, whereby the pieces of data of the respective channels are output through output buffers 44a-44x.
The received signal is also sent to frame synchronization pattern detector 45 through shift register 40. A frame synchronization pattern is here detected, frame synchronization circuit 46 establishes the frame synchronization, and the clock synchronized with the detected frame synchronization pattern is output from clock generator 41.
The conventional frequency-division-multiplex coherent optical communicate system shown in FIG. 1 malfunctions when, for example, a certain channel other than the channel to be selected is in a silent state or when the receiving level is temporarily lowered to prevent the beat detector from detecting a beat signal, and cannot recover to the normal condition unless the system is reset. Moreover, the system as a whole is made complicated due to the necessity for a beat detector.
In addition, a method of inserting a channel number merely enables identification of the channel currently received, and all channels existing between the channel being received and the target channel must be received sequentially and the target channel number must be compared with a received channel number until the target channel has been selected.
In the time-division-multiplex signal transmission system explained with reference to FIGS. 3 and 4, a demultiplexing operation is performed in all channels in the receiving side and thereafter channel selection is carried out after the D/A conversion, FM modulation and frequency-division multiplex for each channel. This makes the scale of circuits very large.
In addition, the quality of a received image is deteriorated due to the troublesome processing including D/A conversion followed by FM modulation, FDM, FDM signal demultiplexlng and FM modulation.